This is yet another thing in the series of highly specific solutions for problems probably nobody else has, but maybe it can serve as inspiration or be adapted for similar cases.
We have a HPE Proliant DL165 server at work that we got from another company that went bust in the same building. Pretty old but still usable for simple tasks. The original caddies for mounting the hard drives were gone however. We only had one dummy caddy in an unused slot. Initially we just shoved the loose disks into their connectors, but one of the disks often dropped out of the RAID due to vibrations causing a glitch in the connection. Who could have guessed that relying on SATA connectors alone does not provide for a secure mount?
Instead of trying to find the now almost antique official caddies for sale, I used the dummy to model minimal brackets that need to be attached to the 3.5" disks with ordinary mounting screws. It doesn't look as pretty as full caddies but works just as well, and offers great airflow.
When we had to replace some of the disks, obviously it made more sense to use SSDs, so I made an adapter to fit a 2.5" SSD into the 3.5" tray. The same 4 screws as for mounting a HDD are used to screw the brackets to the adapter. The SSD can be mounted with two 6 mm M3 countersunk screws. There are two 3 mm holes in the bracket as well but normally do not need to be used, the disk can be securely mounted and should be quite level with the left screws only.
If you want the SSD to be really sturdily mounted, you could insert some short pegs in its right screw holes, or use sawn-off M3 screws (you could cut a slot in them to turn them into tiny screws). Just make sure nothing sticks out more than a millimeter beyond the surface of the bracket, or it will be impossible to slide the tray into the bay.
When printing this, the only thing you really should take care of, is that the top of the rails is printed as close to 4.10 mm as possible. If it is much thicker, you will have a hard time getting the disks in or out of the slots, and if it is much thinner, they will be sloppy. Because I am able to level my build plate pretty accurately, all I had to do was set the first layer height to 0.3 mm, and the other 0.2 mm layers did the rest.
By the way, avoid using the EVO 860 in this server, it seems to have problems with the SATA controller (at least in Ubuntu 16.04). I switched to an older EVO 840 instead.
3D Eksperten PETG
Added SSD adapter, updated right bracket to allow securing the adapter.